1. Field of Invention
The present invention relates to an improved apparatus for measuring the circulating blood volume.
2. Related Art
The circulating blood volume is an important piece of biological information for medical diagnosis. This has conventionally been measured by a method that comprises injecting a dye that is slow in clearing from blood vessels, taking a blood sample at the point of time when the dye has been distributed uniformly throughout the blood in the whole body, measuring the dye density in the blood, and calculating the circulating blood volume from the measured dye concentration.
This method, however, has had two major disadvantages. First it requires lots of steps and time to perform one cycle of measurement; second, the residual dye in blood precludes frequent measurements. If a fast dye is used that is cleared rapidly out of the blood vessels, it is necessary to perform frequent post-injection blood sampling and a corresponding number of measurements must be made to know the dye concentration in the blood samples. Furthermore, the precision of measurements depends on the frequency of blood sampling. Because of these limitations, the use of fast dyes is by no means a practical approach.
The recent advances in electronics have made it possible to achieve noninvasive, continuous and precise measurements of dye concentration in blood by applying the principle of pulse oximetry, in which the ratio between the density of two light absorbers in arterial blood is determined on the basis of the pulsation of light transmitted through a living tissue. To measure the dye density in the blood by this method, the ratio between the densities of hemoglobin and an injected dye is first determined and then multiplied by the separately measured hemoglobin in the blood to determine the absolute value of dye density in arterial blood. The dye dilution curve, or the time-dependent changes in the absolute value of the dye density, has a definite straight line when expressed in a semi-logarithmic graph. Extrapolating this straight line to the dye injection time gives a dye density in the blood that would be obtained if the dye were distributed uniformly in total blood without being cleared from blood vessels. The thus determined dye density is named its initial dye density. Dividing the amount of the injected dye by its initial density will give circulating blood volume.
This method provides for frequent repetitions of measurement by injection of a dye that has only a short lifetime in blood. This method also enables the measurement of the clearing ability of an organ that performs selective excretion of the dye used or the blood flow in that organ.
However, this method which relies upon the principle of pulse oximetry is not free from problems. To meet the need for measuring the light transmitted through a living tissue, the site of measurement is limitted to peripheral such as an earlobe or a fingertip. Further, the dye injection site is often peripheral such as an antecubital vein and, hence, it takes a long time for the injected dye to travel from the injection site to the site of measurement. The length of this time is particularly long if massive bleeding occurs to reduce the circulating blood volume. If, under such circumstances, the straight line obtained by logarithmic transformation of the dye dilution curve is extrapolated to the injection time, the measured initial dye density will differ greatly from the actual value, creating a substantial error in measurement. With a view to eliminating this error, Haneda et al. proposed in 1986 a method for extrapolation to the dye appearance time T.sub.a (Tohoku Journal of Experimental Medicine 1986, 148 page 49-56 "A method for measurement of Total circulating blood volume using indocyanine green"). This method enables elimination of the error dependent difference in the lapse of time from T.sub.0 (dye injection time) to T.sub.a (dye appearance time). For instance, the injected dye will appear after the lapse of 5 to 10 seconds when measurement is conducted at an earlobe whereas a time of 10 seconds to 2 minutes lapses when measurement is conducted at a fingertip. Nevertheless, the dye injected into the blood will mix with the blood and diffuse both forward and backward of that portion of blood. Hence; the most rapidly advancing portion of the injerted dye concentration will advance ahead of the center of the injected dye concentration as it approaches a measuring site. This phenomenon is pronounced if the peripheral blood circulation is inefficient and the time from T.sub.a to the time which is the center of the population of dye appears will sometimes be as great as 30 seconds or more if measurement is conducted at a fingertip. Thus, great errors have occasionally occurred even if extrapolation to the dye appearance time T.sub.a is made.